In recent years the use of high-strength martensitic stainless steels for light structural applications in the transportation industry has increased markedly. In this application, the toughness or resistance of the steel to catastrophic crack propagation is an important property. This has led to development of low-carbon martensitic grades (less than about 0.05% C) with good notch toughness.
One of the steels considered for the above application is a low carbon, 0.03% max., martensitic stainless steel containing 12% chromium, 4% nickel and 0.4% titanium. See for example U.S. Pat. No. 2,397,997, Whyche et al.; and U.S. Pat. No. 3,288,611, Lula et al. These steels are air hardening and can be tempered to various combinations of strength and toughness. Metallurgically, the titanium in the above steel is added to serve as a strong carbide former to tie-up carbon and nitrogen and ensure a low interstitial (essentially interstitial free) martensitic microstructure with good toughness. Nickel is added to prevent delta ferrite formation at high temperatures since such ferrite is deleterious to the toughness of martensitic stainless steels. Although the commercial composition of such steels may of course vary somewhat, the above recited specific composition is claimed to the optimum for achieving an excellent combination of strength and toughness with the leanest possible alloy content in a low carbon martensitic steel..sup.1 FNT .sup.1 G. N. Aggen, C. M. Hammond, and R. A. Lula, "New Martensitic Stainless Steels", Advances in the Technology of Stainless Steels and Related Alloys, ASTM Special Technical Publication 369, Philadelphia, pp. 40-46.